1. Field of the Invention
This invention relates to liquid-cooled cold plates and to the use of porous media, preferably in the form of an aluminum porous matrix, to improve the design of liquid-cooled cold plates, primarily for use in removing waste heat from microwave modules in phased-array-radars.
2. Brief Description of the Prior Art
Currently there are two basic flow options for liquid-cooled cold plates used in thermal management systems for phased-array-radars. A series-flow arrangement cools each cold plate, with modules containing the electronics secured thereto, in series (i.e., one after the other). This results in a large temperature difference between like-components in successive modules since the temperature of the cooling fluid increases along its travel path due to the extraction of heat from the modules via the cold plate. The large temperature difference adversely affects the electrical performance of the array since like components at different locations in the cooling fluid travel path will be at different temperatures and therefore display different electrical characteristics. The solution to this problem in the prior art requires sophisticated, computerized calibration techniques to compensate for these differences.
A parallel-flow arrangement, if attainable, would cool each module in parallel with all other modules in the array, resulting in a uniform temperature between modules and optimum array electrical performance. For most phased-array-radar arrays, however, the scale of the array is small and prior art attempts to utilize parallel-flow cold plates have been unsuccessful because the changes in flow direction required by the small scale and the lack of adequate plenum space result in poorly distributed coolant flow. This poorly distributed flow results in excessive temperature differences between modules, the very problem that the parallel flow concept was intended to solve.
Typically, cold plates used to provide thermal management for phased-array radar modules are constructed by placing a small thickness (typically 0.040 inches) of lanced-offset finstock between two thin (approximately 0.40 inches) aluminum cover plates and vacuum-brazing them together to form a unified assembly. The finstock has approximately 15 to 20 fins per inch which creates a large number of smaller flow passageways. This increases the convective heat transfer coefficient between the cover plates and the liquid coolant flowing between the plates. The finstock also increases the available surface area for heat transfer. The combination of the increase in the heat transfer coefficient and the increase in the surface area available for heat transfer creates an enhancement to the heat transfer, which results in a decrease in the temperature of the aluminum cover plates.
The microwave modules generally contain a thin-film electrical network and electronic components which dissipate heat as they generate and process microwave signals. Component, module, and array reliability are a direct function of component junction temperatures within the modules. The heat generated within the thin-film circuits is conducted to the base of the module which is mounted (screwed, soldered, or epoxied) to the top and bottom surfaces of the cold plate. When the liquid coolant is circulated through the coldplate internal passageways, module and component waste heat is transferred to the flowing coolant and transported away from the array. The more efficient the transfer of heat to the coolant stream, the more reliable the array performance. Efficiency is measured in terms of the temperature difference between the fluid temperature and resulting component temperatures. The smaller the temperature difference, the higher the efficiency. Equally important is the temperature gradient between the modules. For calibrated and stable array operation, the module-to-module difference in temperature should be minimized. In the ideal case, similar components in all modules will have the same operating temperature. The desire is to approach idealized conditions as closely as possible.